Determining the angular rotation rate of a rotating body plays an important role in navigation guidance and control systems. For example, in Inertial Navigation System (INS), it is required to determine the angular accelerations of a vehicle and, thus, the angular orientations thereof. Determining the angular rotation rate may further allow, for example, determining the linear acceleration of a wheeled vehicle (e.g., by attaching an angular velocity sensor to a wheel).
When a mass moves on a rotating body, toward or away from the axis of rotation, a force is exerted perpendicular to the direction of motion. This force is known as the Coriolis force. The magnitude of the Coriolis force is proportional to the angular rotation rate of the body. Thus, the angular rate of rotation (i.e., angular velocity) can be determined by measuring the Coriolis force.
A vibratory gyroscope is a device for determining the angular rotation rate of a body by measuring the Coriolis force. A vibratory gyroscope includes a planar ring suspended by beams coupled with a central support above a substrate. These beams function as springs in any ring-plane (i.e., lateral) direction. Thus, the ring is free to oscillate in any ring-plane direction. It is noted that the ring may oscillate in out of ring-plane directions as well. However, the out of ring-plane oscillation frequencies are substantially higher than the in-plane oscillation frequency.
Reference is now made to FIG. 1A, to FIG. 1B and to FIG. 1.C, which are schematic illustrations of a vibratory gyroscope generally referenced 10, which is known in the art. Vibratory gyroscope 10 includes a ring 14, a plurality of beams 16 and a central support 18. Central support 18 is an integral part of a substrate 12. Beams 16 couple ring 14 to central support 18. It is noted that beams 16 are coupled only with ring 14 and central support 18, thus, ring 14 and beams 15 are suspended above substrate 12 as shown in FIG. 1B. Beams 16 function as springs in a ring-plane direction, represented by arrows 52. Consequently, ring 14 is free to oscillate in any lateral direction represented by arrows 52.
Vibratory gyroscope 10 further includes capacitive actuators 20, 28, 36 and 44, and capacity sensors 24, 32, 40 and 48. Capacitive actuators 20, 28, 36 and 44 and capacity sensors 24, 32, 40 and 48 are capacitively coupled with ring 14. Bond pads 22, 26, 30, 34, 38, 42, 46 and 50 are coupled with capacitive actuators 20, 28, 36 and 44, capacity sensors 24, 32, 40 and 48, respectively and with external circuitry (not shown). Capacitive actuators 20 and 36 are placed opposite each other on an axis that passes through the center of ring 14. Capacitive actuators 28 and 44 are placed on an axis that passes through the center of ring 14 and that is perpendicular to the axis of capacitive actuators 20 and 32. Capacity sensors 24, 32, 40 and 48 are placed in a similar way to capacitive actuators 20, 28, 36 and 44 although the axes of capacity sensors 24, 32, 40 and 48 are at a forty five degree angle relative to the axes of the capacitive actuators 20, 28, 36 and 44.
In operation, capacitive actuators 20, 28, 36 and 44 are pulsed at the resonant frequency of ring 14. Capacitive actuators 20 and 36 are pulsed in phase with each other. Capacitive actuators 28 and 44 are pulsed in phase with each other and out of phase with capacitive actuators 20 and 36. Consequently, an electric field, and thus an electric force is formed between each of capacitive actuators 20, 28, 36 and 44 and ring 14. Thus, ring 14 vibrates in the direction of arrows 64, 66, 68 and 70. This vibration will be referred to hereinafter as “elliptic mode vibration”. The natural frequency of the elliptic mode vibrations will be referred to hereinafter as “elliptic mode frequency”. Furthermore, hereinafter, when the ring is in the elliptic mode vibration, the ring is said to “elliptically vibrate”. When no other forces, other than the electric force, act on ring 14 (i.e., the ring elliptically vibrates), four nodes 52, 54, 53, and 58 are formed on the perimeter of ring 14, whereat the ring substantially does riot move. It is noted that capacity sensors 24, 32, 40 and 48 are placed in proximity to nodes 36, 38, 40 and 42, respectively.
When the ring rotates, a Coriolis force acts on ring 14. Consequently, nodes 52, 54, 56, and 58 radially vibrate at an amplitude proportional to the rate of rotation, and the capacitance between the ring and capacity sensors 24, 32, 40 and 48 changes accordingly. Capacitive sensors 24, 32, 40 and 48 sense this change in capacitance. According to this change in capacitance, it is possible to determine the Coriolis force, and hence the rate of rotation of ring 14.
U.S. Pat. No. 5,225,231, to Varnham et al, entitled “Vibrating Planar Gyro”, directs to a vibrating planar ring or hoop-like structure suspended in space by a suitable support mount for detecting turning rate, linear acceleration and angular acceleration. Turning rate is sensed by detecting vibrations coupled by Coriolis forces. The linear acceleration and the angular acceleration are sensed by lateral, vertical and rocking movement of the entire ring or hoop-like structure, within the mount thereof. The resonator (i.e., the vibrating structure) is formed on a center plate and lies between an upper plate and a lower plate. The resonator is formed of a stable material such as glass, silicon or quartz wafer or sheet. The upper and the lower plates may be formed of glass, quartz or fused silica.
Two sets of transducers are located above and below the resonator, respectively. Each transducer includes two sets of concentric electrode strips, one set located on the upper or lower surface of the resonator, and the other set located on the upper or lower plates. Each transducer produces an output signal which is representative of the distance between its respective sets of the electrode strips. Two transducers excite the resonator at a resonate frequency along a determined primary axis in the plane of the resonator. The output of the transducers gives an indication of acceleration and turning rate.
U.S. Pat. No. 5,450,751, to Putty et al., entitled “Microstructure for Vibratory Gyroscope”, directs to a microstructure for a vibratory gyroscope of the variety sensing rotation about an axis. Eight, equally distributed semicircular or “S” shaped spokes couple a ring to a hub. The hub is coupled to a silicon substrate base. The spokes and the rings are free standing away from the base.
A multiplicity of charge conductive sites is disposed symmetrically around the outer perimeter of the ring and adjacent thereto. The arrangement of charge conductive sites adequate drive and detection of a resonant standing wave pattern in the ring. The Base may also includes prefabricated circuitry as a monolithic integrated circuit, a portion of which may be ohmically coupled to the microstructures via exposed metallization sites.
In the publication to Putty et al., it is desirable to increase the height and the diameter of the ring and to decrease the width of the ring. Consequently, the natural (i.e., the resonant) frequency of the ring is decreased, yielding increased sensitivity of the ring. However, the natural frequency of the ring should by kept below out-of-plane natural frequencies and above the frequencies of external vibrations (e.g., the vibrations of a motor vehicle when sensing rotations in this motor vehicle).
In the publication to Putty et al., the microstructure (i.e., the ring and the spokes) is formed by constructing a mold from polyimide on a passivation layer made of silicon nitride of oxide. When the mold is in place, a barrett sultamate nickel process is used to form a nickel microstructure. When the mold and the passivation layer are removed, the microstructure is left freestanding.
U.S. Pat. No. 6,282,958, to Fell at al., entitled “Angular Rate Sensor”, directs to an angular rate sensor suitable for sensing motion about at least one axis. The angular rate sensor to Fell et al. includes a resonator having a ring or hoop-like shape structure, flexible support beams, a boss and a base. The boss is coupled with the base. The flexible support beams couple the ring structure with the boss, so that the ring resonator structure is spaced from the boss and the base. The angular rate sensor further includes electrostatic drive means for causing the resonator to vibrate at cos(20) carrier mode, and electrostatic sensing means for sensing movement of the resonator. When the sensor is rotated about the Z axis, Coriolis force couples energy into the response mode with amplitude and it is directly proportional to the applied rotation rate. This motion is sensed by the sensing means (i.e., pick-off elements).
According to the publication to Fell et al., the ratio of the lateral thickness of the resonator, to the width between the outer periphery of the resonator and an adjacent drive or pick-off element must be between 10:1 to 40:1 to maximize the capacitance between the resonator and the electrostatic elements.
U.S. Pat. No. 6,471,883, to Fell et al., entitled “Method of Manufacturing a Vibrating Structure Gyroscope”, directs to a method of manufacturing a vibrating structure gyroscope having a silicon substantially planar ring vibrating structure and capacitive means for imparting drive motion to, and sensing motion of the vibrating structure. The silicon vibrating structure includes a substantially planar ring resonator, support legs and a central hub. The support legs couple the resonator with the central hub. The hub is coupled with a plate like glass or silicon substrate. Thus, the resonator structure mounted by a hub above the substrate cavities provides unrestricted oscillation of the ring structure. The gyroscope includes capacitive drive means for imparting drive motion to the ring resonator and capacitive sensing means for sensing and picking off motion of the ring resonator.
The method according to the publication to Fell et al., includes the steps of: depositing a first layer of photoresist material onto one surface of the plate like glass or silicon substrate and exposing selected areas of the substrate. Etching the exposed areas of the substrate to form cavities therein and stripping the remaining first layer photoresist material from the cavitated substrate. Attaching a layer of silicon to the cavitated substrate. Depositing a layer of aluminum on the surface of the silicon layer. Depositing a second layer of photoresist material on to the outermost surface of the aluminum layer with respect to the silicon layer and exposing selected areas of the aluminum layer. Etching said exposed areas of the aluminum layer to leave on the silicon layer regions of aluminum providing bond pads for grounding the screen layer, bond pads forming connection points for the capacitive drive and sensing means, and bond pads for electrical connection to the silicon substantially planar ring vibrating structure. Depositing a third layer of photoresist material onto the silicon layer over the remaining deposited aluminum layer regions and exposing selected areas of the silicon layer. Performing deep reactive ion etching of the exposed selected areas of the silicon layer to form, from the silicon layer, the capacitive drive and sensing means, and electrically isolating each of the capacitive drive and sensing means, screen layer and ring vibrating structure from one another.